Positive seals such as roll-sock seals and bellows have been used for piston rod applications but these are limited to short strokes and their short life is a major disadvantage.
The majority of piston rod seals can be classified as lubricated seals in which the interface between the seal and the piston rod is lubricated to reduce friction, wear and heat generation. The presence of the lubricant in the interface also reduces the leakage of gas from the high pressure chamber. The lubricant, usually oil, is applied directly to the surface of the piston rod on the low pressure side of the seal. While the lubricant plays an essential role in providing an effective gas seal in the majority of applications it is very undesirable to have any significant accumulation of lubricant in the high pressure gas chamber. A typical example is the main seals or piston rod seals in a reciprocating Stirling engine. The primary function of these seals is to minimize the leakage of gas (usually hydrogen) from the working cycles into the crankcase. An equally important requirement is that they prevent oil from the crankcase from entering the working gas region where it will contaminate the engine and deteriorate its performance.
To meet the requirements for negligible oil leakage the majority of lubricated seals incorporate a section which acts as a scraper. The scraper is intended to remove most of the lubricant from the piston rod and allow only a minimal quantity to enter the seal/piston rod interface. The effectiveness of the scraping action in removing lubricant is generally unpredictable and this can have a very strong influence on the life of the seal. If the scraper allows too little lubricant to pass into the seal/rod interface there will be an incomplete lubricant film. This will produce a less effective gas seal and there will be at least partial sliding contact between the rod and the seal. This contact will increase the friction and wear and accelerate the failure of the seal. If the scraper allows too much lubricant to pass into the rod/seal interface there is a strong possibility that the lubricant will contaminate the high pressure gas chamber unless the seal is designed to ensure that all the oil passes back through the rod/seal interface when the stroke is reversed.
One example of a lubricated seal is described in U.S. Pat. No. 4,251,081. The basic features of the seal are shown in FIG. 1. The seal 12 made of graphite reinforced polytetrafluoroethylene surrounds the piston rod 6. Oil is supplied to the surface of the piston rod through an annular groove 16 on the low pressure side of the seal. The seal is loaded against the surface of the piston rod by a compression spring system 13 and 14. The sharp, lower edge of the seal 18, acts as a scraper. On the high pressure side the seal is provided with a tubular extension 15 having a slightly increasing inner diameter in the direction towards the high pressure chamber. On the downstroke any oil present on the surface of the rod above the seal passes into the converging passage between the rod and the seal extension. The geometry of the passage combined with the motion of the rod produces a pumping effect and as a result the oil is returned to the low pressure side of the seal.